Children of Pyxis

The Children of Pyxis were diminutive assassins deployed by Skorpios to kill the Doctor. They were inflicted with a condition called the Pyxis tragedy, causing them to operate on negative time. This meant as soon as they left one place, the time stream was fooled to believe that they had never been there. Thus, no one remembered them, even reckoning their existence a myth. They also aged backwards, and believed they were bringing life and creating when they were actually killing and destroying. The Doctor and Romana were able to remember them due to being time sensitive.

The Children of Pyxis followed the Doctor to the micro-universe where witnesses and the Lost Jury of Court 666Z were kept. They killed many of the villagers there, but the Doctor lured them to the jury room where time ran slower and held them in a sonic field with his sonic screwdriver and K9. Romana later brought a sonic field generator to keep them in place indefinitely until they would be "unborn". (AUDIO: The Doomsday Contract)